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Communicating with Workflows

DBOS provides a few different ways to communicate with your workflows. You can:

Workflow Messaging and Notifications​

You can send messages to a specific workflow. This is useful for signaling a workflow or sending notifications to it while it's running.

DBOS Steps

Send​

DBOS.send<T>(destinationID: string, message: T, topic?: string): Promise<void>;

You can call DBOS.send() to send a message to a workflow. Messages can optionally be associated with a topic and are queued on the receiver per topic.

You can also call send from outside of your DBOS application with the DBOS Client.

Recv​

DBOS.recv<T>(topic?: string, timeoutSeconds?: number): Promise<T | null>

Workflows can call DBOS.recv() to receive messages sent to them, optionally for a particular topic. Each call to recv() waits for and consumes the next message to arrive in the queue for the specified topic, returning null if the wait times out. If the topic is not specified, this method only receives messages sent without a topic.

Messages Example​

Messages are especially useful for sending notifications to a workflow. For example, in the e-commerce demo, the checkout workflow, after redirecting customers to a secure payments service, must wait for a notification from that service that the payment has finished processing.

To wait for this notification, the payments workflow uses recv(), executing failure-handling code if the notification doesn't arrive in time:

@DBOS.workflow()
static async checkoutWorkflow(...): Promise<void> {
  ...
  const notification = await DBOS.recv<string>(PAYMENT_STATUS, timeout);
  if (notification) {
      ... // Handle the notification.
  } else {
      ... // Handle a timeout.
  }
}

A webhook waits for the payment processor to send the notification, then uses send() to forward it to the workflow:

static async paymentWebhook(): Promise<void> {
  const notificationMessage = ... // Parse the notification.
  const workflowID = ... // Retrieve the workflow ID from notification metadata.
  await DBOS.send(workflowID, notificationMessage, PAYMENT_STATUS);
}

Reliability Guarantees​

All messages are persisted to the database, so if send completes successfully, the destination workflow is guaranteed to be able to recv it. If you're sending a message from a workflow, DBOS guarantees exactly-once delivery. If you're sending a message from normal TypeScript code, you can specify an idempotency key for send to guarantee exactly-once delivery.

Workflow Events​

Workflows can publish events, which are key-value pairs associated with the workflow. They are useful for publishing information about the status of a workflow or to send a result to clients while the workflow is running.

DBOS Steps

setEvent​

DBOS.setEvent<T>(key: string, value: T): Promise<void>

Any workflow can call DBOS.setEvent to publish a key-value pair, or update its value if has already been published.

getEvent​

DBOS.getEvent<T>(workflowID: string, key: string, timeoutSeconds?: number): Promise<T | null>

You can call DBOS.getEvent to retrieve the value published by a particular workflow ID for a particular key. If the event does not yet exist, this call waits for it to be published, returning null if the wait times out.

You can also call getEvent from outside of your DBOS application with DBOS Client.

Events Example​

Events are especially useful for writing interactive workflows that communicate information to their caller. For example, in the e-commerce demo, the checkout workflow, after validating an order, directs the customer to a secure payments service to handle credit card processing. To communicate the payments URL to the customer, it uses events.

The checkout workflow emits the payments URL using setEvent():

@DBOS.workflow()
static async checkoutWorkflow(...): Promise<void> {
  ...
  const paymentsURL = ...
  await DBOS.setEvent(PAYMENT_URL, paymentsURL);
  ... 
}

The HTTP handler that originally started the workflow uses getEvent() to await this URL, then redirects the customer to it:

static async webCheckout(...): Promise<void> {
  const handle = await DBOS.startWorkflow(Shop).checkoutWorkflow(...);
  const url = await DBOS.getEvent<string>(handle.workflowID, PAYMENT_URL);
  if (url === null) {
    DBOS.koaContext.redirect(`${origin}/checkout/cancel`);
  } else {
    DBOS.koaContext.redirect(url);
  }
}

Reliability Guarantees​

All events are persisted to the database, so the latest version of an event is always retrievable. Additionally, if getEvent is called in a workflow, the retrieved value is persisted in the database so workflow recovery can use that value, even if the event is later updated.

Workflow Streaming​

Workflows can stream data in real time to clients. This is useful for streaming results from a long-running workflow or LLM call or for monitoring or progress reporting.

DBOS Steps

Writing to Streams​

DBOS.writeStream<T>(key: string, value: T): Promise<void>

You can write values to a stream from a workflow or its steps using DBOS.writeStream. A workflow may have any number of streams, each identified by a unique key.

When you are done writing to a stream, you should close it with DBOS.closeStream. Otherwise, streams are automatically closed when the workflow terminates.

DBOS.closeStream(key: string): Promise<void>

DBOS streams are immutable and append-only. Writes to a stream from a workflow happen exactly-once. Writes to a stream from a step happen at-least-once; if a step fails and is retried, it may write to the stream multiple times. Readers will see all values written to the stream from all tries of the step in the order in which they were written.

Example syntax:

async function producerWorkflowFunction() {
  await DBOS.writeStream("example_key", { step: 1, data: "value1" });
  await DBOS.writeStream("example_key", { step: 2, data: "value2" });
  await DBOS.closeStream("example_key"); // Signal completion
}

const producerWorkflow = DBOS.registerWorkflow(producerWorkflowFunction);

Reading from Streams​

DBOS.readStream<T>(workflowID: string, key: string): AsyncGenerator<T, void, unknown>

You can read values from a stream from anywhere using DBOS.readStream. This function reads values from a stream identified by a workflow ID and key, yielding each value in order until the stream is closed or the workflow terminates.

You can also read from a stream from outside a DBOS application with a DBOS Client.

Example syntax:

for await (const value of DBOS.readStream(workflowID, "example_key")) {
  console.log(`Received: ${JSON.stringify(value)}`);
}